There are many different modulation schemes used to modulate a Radio Frequency (RF) carrier or an Intermediate Frequency (IF) carrier with a lower frequency modulation signal. There are different advantages and disadvantages to each of the commonly used methods. There are also many different wireless formats or standards used within the wireless product marketplace today. These standards differ not only in modulation technique used, but also in the bandwidth utilized by the wireless system, the RF bandwidth of a single channel, and the use, or not, of various spread spectrum techniques, such as CDMA (Code Division Multiple Access), frequency hoping techniques, or, most recently gaining in popularity, OFDM (Orthogonal Frequency Division Multiplexing). An idealized goal of a Software Defined Radio (SDR) system is to be able to transmit and receive using any of these techniques and to be able to switch between them by merely changing the software code running on such an SDR system.
Cognitive Radio Systems, as currently envisioned within the R&D community, include plans for flexible transceiver's, which can adjust to band utilization variations as needed, changing frequency bands, modulation techniques, and transmission bandwidths as required to make best use of the current RF environment. This discussion continues, yet currently there is not even a cost effective or efficient method to implement a fully flexible SDR.
Given the lack of fully flexible SDR technology, each of the many differing modulation techniques and wireless standards have historically required different customized analog front-end receiver blocks and customized back-end analog transmitter blocks for each bandwidth, modulation technique, or wireless standard accommodated. The concept of SDR has often been put forth with the promise of a single circuit block that could, under software control, be able to operate and provide competitive performance, while working with any of the current wireless transmission schemes. Yet this promise remains unfulfilled.
The roadblocks to achieving fully flexible SDR solutions have been many. The use of IF stages, creating difficult to manage spurs, and image frequency artifacts, also create overly complex matrices of usability limitations. Each combination of center frequency, bandwidth, and modulation scheme has required specific design attention despite a theoretically programmable feature selection. Quadrature modulation and all schemes which make use of phase variation add a great deal of complexity to both reception and transmission. Maintaining orthogonality and minimizing mismatches between the I and Q channels are ongoing challenges. Conventional zero IF approaches to the SDR challenge attempt to simplify the IF complexities, but compound the I/Q mismatch and orthogonality issues and typically degrade noise performance as well. Real solutions to the SDR challenge have seemed perennially imminent, while remaining ethereal.